[PATCH net-next v5 3/8] TUN, TAP & vhost_net: Stop netdev queue before reaching a full ptr_ring
From: Simon Schippers <hidden>
Date: 2025-09-22 22:17:09
Also in:
kvm, lkml, virtualization
Subsystem:
networking drivers, the rest, tun/tap driver · Maintainers:
Andrew Lunn, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds, Willem de Bruijn, Jason Wang
Stop the netdev queue ahead of __ptr_ring_produce when __ptr_ring_full_next signals the ring is about to fill. Due to the smp_wmb() of __ptr_ring_produce the consumer is guaranteed to be able to notice the stopped netdev queue after seeing the new ptr_ring entry. As both __ptr_ring_full_next and __ptr_ring_produce need the producer_lock, the lock is held during the execution of both methods. dev->lltx is disabled to ensure that tun_net_xmit is not called even though the netdev queue is stopped (which happened in my testing, resulting in rare packet drops). Consequently, the update of trans_start in tun_net_xmit is also removed. Co-developed-by: Tim Gebauer <redacted> Signed-off-by: Tim Gebauer <redacted> Signed-off-by: Simon Schippers <redacted> --- drivers/net/tun.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-)
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 86a9e927d0ff..c6b22af9bae8 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c@@ -931,7 +931,7 @@ static int tun_net_init(struct net_device *dev) dev->vlan_features = dev->features & ~(NETIF_F_HW_VLAN_CTAG_TX | NETIF_F_HW_VLAN_STAG_TX); - dev->lltx = true; + dev->lltx = false; tun->flags = (tun->flags & ~TUN_FEATURES) | (ifr->ifr_flags & TUN_FEATURES);
@@ -1060,14 +1060,18 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) nf_reset_ct(skb); - if (ptr_ring_produce(&tfile->tx_ring, skb)) { + queue = netdev_get_tx_queue(dev, txq); + + spin_lock(&tfile->tx_ring.producer_lock); + if (__ptr_ring_full_next(&tfile->tx_ring)) + netif_tx_stop_queue(queue); + + if (unlikely(__ptr_ring_produce(&tfile->tx_ring, skb))) { + spin_unlock(&tfile->tx_ring.producer_lock); drop_reason = SKB_DROP_REASON_FULL_RING; goto drop; } - - /* dev->lltx requires to do our own update of trans_start */ - queue = netdev_get_tx_queue(dev, txq); - txq_trans_cond_update(queue); + spin_unlock(&tfile->tx_ring.producer_lock); /* Notify and wake up reader process */ if (tfile->flags & TUN_FASYNC)
--
2.43.0